Whirling disease (WD) is an emerging parasitic disease of salmonids that is increasing in severity and geographic range. Whirling disease is caused by the myxosporean parasite, Myxobolus cerebralis, and can effect significant mortality in wild and cultured salmonid populations. Myxobolus cerebralis was recently detected in Yellowstone National Park (YNP) where it may be causing native Yellowstone cutthroat trout (YCT) to decline. Myxobolus cerebralis exploits the aquatic oligochaete, Tubifex tubifex, as its primary host and spores released by T. tubifex are infective to salmonid fish. The aim of this study was to assess WD risk for YCT populations in YNP by focusing on the disease source, T. tubifex, which had not previously been characterized. My objectives were to characterize T. tubifex populations and dynamics of M. cerebralis infections in T. tubifex and to establish factors associated with M. cerebralis infections in T. tubifex. In addition, I examined relationships between infection dynamics in T. tubifex and transmission to fish hosts (WD risk).

In Pelican Creek, T. tubifex and M. cerebralis infected T. tubifex were widely distributed and abundant. Infected T. tubifex were most abundant in reaches characterized by intermediate geothermal influence. However, WD risk was high in all reach types, which indicated that low parasite success in the oligochaete host in reaches with high or no geothermal influence did not translate into reduced WD risk in these reaches. In tributaries throughout YNP, susceptible T. tubifex were widely distributed but experimental and field data suggest M. cerebralis -infected T. tubifex may be unable to survive in all tributaries where uninfected T. tubifex were found. In particular, environmental factors influenced by confinement, including proportions of coarse and fine substrates, may preclude establishment of M. cerebralis in tributaries in YNP. Thus, environmental features, rather than oligochaete host factors, may be most influential for M. cerebralis dynamics in T. tubifex and WD risk to fish in YNP. While further research is needed to identify specific mechanisms, these results suggest environmental features related to confinement may be useful for assessing WD risk at broad scales when the oligochaete host is characterized by low genetic variability.